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Bacteria: Listeria monocytogenes studies

Problem

Listeria monocytogenes is a Gram-positive, rod-shaped bacterium responsible for listeriosis, a severe food-borne human infection with an overall mortality rate of 30%. It has evolved efficient strategies to survive in the intestine and cross the intestinal, blood–brain and placental barriers, leading to gastroenteritis, septicemia, central nervous system infections, and mother-to-child infections. 
L. monocytogenes spreads from cell to cell aided by the listerial membrane surface protein ActA, encoded by the gene actA. The bacterial membrane-bound ActA protein recruits the host cell’s Arp2/3 protein complex, which helps to polymerize actin filaments at the posterior end of the bacterium. Nucleation with the Arp2/3 complex generates branched arrays of filament that grow towards the membrane to which they are tethered, elongating the actin strand and creating a sort of comet tail propels the bacterium forward. This function allows the bacterium to move throughout the host cell cytoplasm as well as to invade neighboring eukaryotic cells.
Figure 1  ActA protein, tethered to the bacterial membrane, stimulates actin filament nucleation with the Arp2/3 complex; attribution: Benjamin A Smith et al, CC-BY 4.0.
Researchers interested in combating the danger of L. monocytogenes contamination of refrigerated foods performed the following experiments.
Experiment 1
Researchers studied the peculiar property of L. monocytogenes to grow and multiply at lower temperatures than other bacteria. Two types of foodborne bacteria, L. monocytogenes and Escherichia coli, were cultured in a medium and refrigerated at 4 degrees Celsius.  The colonies of the two species were kept separate to prevent any horizontal gene transfer. The number of bacteria was then measured every few weeks and a growth curve was plotted.
Figure 2 L. monocytogenes and E. coli bacteria growth curves
Experiment 2
Researchers investigated CK-0944636, an Arp2/3 inhibitor that binds Arp2/3 within its hydrophobic core and inhibits the formation of the actin comet tail. Researchers cultured different L. monocytogenes strains, with or without actA gene disruption, in media containing freely soluble actin monomers with or without CK-0944636. The results are shown in Figure 2. 
Figure 3 The rates of actin filament polymerization in µM1s1 for three different strains of L. monocytogenes
Data adapted from: “Dortet L, Mostowy S, Louaka AS, Gouin E, Nahori M-A, et al. (2011). Recruitment of the Major Vault Protein by InlK: A Listeria monocytogenes Strategy to Avoid Autophagy. PLoS Pathog 7(8): e1002168.
The growth exhibited in weeks 0–3 of the bacterial growth curves (Figure 2) is attributed to which process?
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